Electron discharge amplifier



May 15, 1934. J. KAAR 1,959,165

ELECTRON DISCHARGE AMPLIFIER Filed Oct. 26, 1932 Fig. 1.

Inventor: Ira J. Kaar,

by Mu /L H is Attorney.

Patented May 15, 1934 UNITED ST ELECTRON DISCHARGE AMPLIFIER Ira J.Kaar, Schenectady, N. Y., assignor to General Electric Company York , acorporation of New Application October 26, 1932, Serial No. 639,630

8 Claims.

My invention has for one of its objects to provide an arrangement forpreventing self-oscillation or reaction between the anode and gridcircuits in an ultra high frequency amplifier by neu-- tralizing theeiiect of the inherent reactances of the cathode, the screen gridelement and the leads connected thereto.

In accordance with my invention I provide means for maintaining thescreen grid and the cathode at the radio frequency potentials underwhich condition maximum advantage is derived from the screening actionof the screen grid.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My in- '20vention itself, however, both as to its organization and method ofoperation together with further objects and advantages thereof, will beunderstood best by reference to the following description taken inconnection with the accompanying drawing in which Fig. 1 discloses apreferred embodiment of my invention, and Figs. 2 and 3 are explanatoryillustrations of my invention.

Referring to Fig. 1 of the drawing, I have illustrated therein a portionof a high frequency amplifier including an electron discharge device ofthe type commonly used in high and ultra high frequency transmitters. Itis of course to be understood that my invention is also applicable tohigh and ultra high frequency amplifiers of low power such as arecommonly used in radio receiving apparatus. The electron dischargedevice 1 is provided with a filamentary cathode 2, a control grid 3,ascreen grid 4 and an anode 5. The screen grid 4 is provided with theusual lead thereto extending throughone terminal of the electrondischarge envelope adjacent to the leads connected to the filamentarycathode 2. A second lead is connected to the screen grid at the otherextremity thereof and this extends to a terminal 6 located at one sideof the envelope. This terminal is usually connected through a largecapacitor (not shown) to the cathode and ground. The amplifier isprovided with a suitable input device or transformer 7, the secondary ofwhich iscOnnected at one terminal to the control grid 3, and at theother terminal to a suitable source of grid biasing voltage which, inturn, is connected to the cathode and ground. The output circuit of theamplifier is provided with a suitable output device or transformer 8,the primary of which is connected at one terminal tothe anode 5 and theother terminal of which is connected to the positive terminal of asuitable source'of anode voltage. The filamentary cathode 2 may beenergized by means of a transformer 9 which is connected to a suitablesource of alternating current. The screen grid 4 is supplied withpositive voltage from a convenient source. A plurality of capacitors 10and 10a are connected in series across the connections to the cathode.The point between the capacitors is connected to the midpoint on thesecondary of the transformer 9 and this is maintained at groundpotential.

The amplifier as described is not suitable for amplification offrequencies above approximately 20 megacycles. This amplifier dependsupon its freedom from self-oscillation or anode circuit to grid circuitreaction upon the screen grid 4 which is interposedbetween the controlgrid 3 and the anode 5, and which is supposedly maintained at cathodepotential for all frequencies. It has been found, however, thatsufficient reactance exists along the length of the filament, the leadsto the filament and the screen grid, and the screen grid itself, topermit the screen grid to assume potentials during high frequencyoperation which will depart from the cathode potential. This results inineffective shielding and in unstable operation of the electrondischarge device, thereby causing self-oscillations or giving rise todegenerative or regenerative coupling between the anode and control gridcircuits. This is caused bythe grid circuit to plate circuit couplingwhich exists when the screen grid is not maintained at the same potential as the cathode at the operating frequencies. I

In accordance with my invention, the screen grid lead-in adjacent to theconductors is main-' tained at alternating current ground and cathodepotential by means of a'capacitor 11 which is connected to the groundand to the cathode. A variable reactor 12 which may be either aninductor or a capacitor is connected between the ground and cathode, andthe terminal 6 which is the connection to the other extremity of thescreen grid 4. In order to compensate for the inherent reactance in theelements of the electron discharge device and the leads thereto, thereactor 12 is adjusted so that its reactance value is equal and oppositeto the algebraic sum of the reactances of the elements and leads at theoperating frequency. This, however, does not imply that the seriescircuit comprising the cathode to screen grid capacity, leads andexternal capacitor 12 are all tuned to series resonance, as

this adjustment results in maximum difference in potential between thecathode and screen grid, and this condition is to be avoided.

Reference may now be had to Fig. 2 wherein I have diagrammatically shownthe cathode 2, the screen grid element 4 and the capacitance A existingbetween these elements. The reactance of the cathode structure itself isindicated by the rectangle B connected in series with the cathode andthe reactance of the leads to the cathode which is indicated by therectangle 0. Usually these reactances are inductive in nature, but theymay be capacitive. The reactance of the screen grid structure and thereactance of the screen grid leads are shown as rectangles D and Econnected in series with the screen grid 4. These two systems areconnected together by the capacitor 11. It will be obvious that if thesum of reactances of the circuit reactances B, C, 11, D and E is notzero, current will pass through the capacitance A, and a difference ofpotential will exist between the cathode and screen grid. If the loopcircuit composed of the reactances A, B, C, 11, D and E is zero, then amaximum current will pass through the capacitance A and a maximumdifference of potential will exist between the cathode and the screengrid. These conditions must be avoided if stable operation of theamplifier is to be obtained.

In accordance with my invention, a variable reactor 12 is inserted atany point in the loop circuit external to the discharge device, as forexample a point between the capacitor 11 and the screen grid reactancesD and E as shown in Fig. 3. When properly adjusted, this reactor 12serves to cancel the effect of the reactances existing in the circuitbetween the cathode 2 and the screen grid element 4 by way of circuit B,C, 11, D and E. The reactor 12 is adjusted so that the algebraic sum ofthe reactances B, C, 11, 12, D and E is zero; thus no current will flowthrough the capacitance between the cathode and screen grid. Under thiscondition the screen grid is maintained at the same radio frequencypotential as the cathode and the desired stable operation of theamplifier will be obtained.

This arrangement permits the amplifier to operate at ultra highfrequencies and it is equally applicable to transmitting and receivingcircuits.

While I have shown and described my invention in connection with certainspecific embodiments, it will of course be understood that I do not wishto be'limited thereto since it is apparent that the principles hereindisclosed are susceptible of numerous other applications, andmodifications may be made in the circuit arrangements or in theinstrumentalities employed without departing from the scope and spiritof my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. The combination with a high frequency amplifier including an electrondischarge device having a cathode, a screen grid element, and leadsconnected thereto, of means connected between said cathode and saidscreen grid element for introducing a reactance having a value equal andopposite to the algebraic sum of the reactances of said leads, saidcathode and said screen grid at the operating frequency of the amplifierwhereby the maximum effect of said screen grid element is obtained.

2. The combination, in a high frequency amplifier, of an electrondischarge device having a cathode, a screen grid element, and leads.connected thereto, and a reactor connected between said screen gridelement and said cathode having such a value of reactance that theinherent reactance of said cathode, said screen grid and the leadsconnected thereto is neutralized at the operating frequency of theamplifier.

3. The combination, in a high frequency amplifier including an electrondischarge device having a cathode, a screen grid element, and leadsconnected thereto, said screen grid element being provided withconnections at either extremity, of means connected between oneextremity of said screen grid element and said cathode for maintainingsaid extremity at radio frequency cathode potential, and means connectedbetween said cathode and said other extremity of said screen gridelement having reactance opposed to the inherent reactances of saidcathode, said screen grid and the leads connected thereto, and of valuesufficient to neutralize said inherent reactances.

4. The combination, in a high and ultra high frequency amplifierincluding an electron discharge device having a cathode, a screen gridelement, and leads connected thereto, said screen grid element beingprovided with connections at either extremity, of means connectedbetween one extremity of said screen grid element and said cathode formaintaining said extremity at the same operating voltage as saidcathode, and means connected between said cathode and said otherextremity of said screen grid element for maintaining said screen gridat the same operating potential as the cathode at the operatingfrequency, said last means having a reactance opposite to, andsufiicient for neutralizing the algebraic sum of, the reactances of saidcathode, said screen grid element and said leads connected thereto.

5. The combination, in a high and ultra high frequency amplifierincluding an electron discharge device having a cathode, a screen gridelement, and leads connected thereto, said screen grid element beingprovided with connections at either extremity, of a capacitor connectedbetween one extremity of said screen grid element and said cathode, anda reactor connected between the other extremity of said screen gridelement and said cathode, said reactor being arranged to provide at theoperating frequency of said amplifier a reactance having a valuesufficient to neutralize the algebraic sum of the reactances of saidcathode, said screen grid element and said lead connected theretowhereby said screen grid element is maintained at the same radiofrequency potential as the cathode at the operating frequency of theamplifier.

6. The combination, in a high frequency amplifier including an electrondischarge device having a screen grid element, a filamentary cathode,and leads connected thereto, said screen grid element being providedwith leads at either extremity, of a by-pass capacitor connected betweenone extremity of said screen grid element and said cathode, and avariable reactor connected between the other extremity of said screengrid element and said cathode, said variable reactor being arranged toprovide at the operating frequency of said amplifier a reactance havinga value equal and opposite to the algebraic sum of the reactances ofsaid cathode, screen grid element and said leads connected theretowhereby the maximum effect of said screen grid element is obtained.

7, In a high frequency amplifier having an tion of an electron dischargeamplifier having a cathode, a screen grid element, and leads connectedthereto, which includes maintaining one end of said screen grid elementat substantially radio frequency cathode potential, and maintaining thealgebraic sums of the reactances of said leads, said cathode and screengrid elements at the operating frequency equal to zero.

IRA J. KAAR.

